Large scale computer systems typically have a number of component parts such as processors, storage devices, and input/output units, and data is regularly transferred among them during the execution of system tasks. In distributed processing systems, which generally employ several cooperating processors to perform system tasks, the problem of communications among the various units of the system can become very complex.
As distributed processing systems become widely used, attention is being given to standardizing the data communication systems which provide communications between the various parts that make up the distributed processing system. One type of communication system which has gained wide acceptance is the carrier sense multiple access-collision detect (CSMA-CD) type of system, exemplified by the Ethernet.RTM., a multipoint access data communication system with collision detection, which is substantially described in Metcalfe et al., U.S. Pat. No. 4,063,220.
The data communication system described by Metcalfe et al. comprises a bit-serial receiver-transmitter network which provides a communication path for a plurality of using devices, such as processors, storage devices, and input output units, that are connected to it. The network is centered around a communicating medium, such as a coaxial cable, to which a plurality of transceivers are tied for communicating across the medium. A plurality of corresponding using devices is connected to the plurality of transceivers by means of associated network interface stages. Each transceiver and interface stage combination constantly senses the data stream passing through the network, the interface stage constantly looking for data packets addressed to it, that is, to its associated using unit.
In addition to providing normal transmitting and receiving functions, the transceiver provides further control functions, utilized for aborting a transmission of data packets from the interface stage, when it senses a collision on the medium--herein also referred to as remote collision--of data transmitted by the associated and another interface stage. Also, the associated interface stage blocks its transmission of data when it senses the presence of other data on the medium.
While being advantageous from the standpoint of simplicity and versatility, conventional Ethernet-type data communication systems suffer from certain disadvantages that limit their application and usefulness. For example, such systems utilize a single communication medium, in terms of providing only a single communication path between any two using devices. The use of a simplex medium adversely affects the availability of the communication system: system reliability is low because the system is brought down by a single failure in the communication medium. Such fault sensitivity in turn dictates the use of a very reliable, and hence very expensive, communication medium to lower the probability of failure, which increases system cost. The use of a simplex medium also demands that the system be shut down to service the communication medium and to connect using units to the medium or to disconnect using units from the medium. Though devices for making in-service taps of the medium have been developed, they often provide connections to the medium which are of questionable reliability and may cause shorts in the medium which lower the medium's reliability.
Also in conventional Ethernet-type systems, each using device is connected to the communication medium at a separate tap via a dedicated transceiver and interface stage combination. Because of the deterioration of the medium's electrical characteristics caused by taps on the medium, the number of taps, and hence the number of using devices that can be connected to the medium, has been limited. Also, the use of a separate dedicated transceiver with each using device has made addition of using devices to the system costly. But, inter alia because of the way in which collision detection is accomplished in such systems--for example through a bit-by-bit comparison of signals transmitted and signals simultaneously detected by a transceiver on the medium--the use of a single transceiver to service more than one using device without radically altering the communications protocol has not been thought possible.